Load following

Load following, also called ramping, is a generic term for the essential grid operation of ramping supply sources up or down in response to changing load or supply conditions. Load following may occur over the course of a few seconds or minutes to several hours depending on the type of resource. Such activity has long been a part of daily grid operations, particularly to meet expected changes in demand throughout the day. Load following typically occurs in response to dispatch instructions from the bulk system operator. Typical load-following resources include hydropower; gas combustion turbines; gas combined-cycle turbines; gas, coal, or oil steam turbines, and in some regions nuclear steam turbines. It may also be provided by curtailment of wind or solar resources. 

An example of load following by a generator